1. Field of the Invention
Aspects of the present invention relate to a method of scanning an access point (AP) during a handoff procedure of a base station in a wireless communication system, a network interface to support the method, and a wireless communication system to enable the method. More particularly, aspects of the present invention relate to a method of quickly scanning an AP during a handoff procedure in a wireless communication system, a network interface to support the method, and a wireless communication system to enable the method in which the wireless communication system includes at least one AP and at least one mobile station.
2. Description of the Related Art
As Institute of Electrical and Electronics Engineers (IEEE) 802.11 wireless local area networks (WLANs) are widely deployed and emergence of new services accelerates, support for real-time interactive applications, such as Voice over WLAN (VoWLAN), becomes increasingly necessary. Since mobility of user terminals is an important issue in an All-Internet Protocol (IP) environment, there is an increased need to provide seamless services to a mobile station moving in a wireless communication system. Thus, technology to enhance quality of a wireless communication service by reducing latencies and failures occurring during a handoff becomes indispensable.
The handoff according to IEEE 802.11 WLANs consists of three components, scanning, authentication, and re-association. (1) Scanning is a process to discover access points (APs) suitable for handoff and acquire AP information of the discovered APs; (2) authentication is a process to identify an individual mobile station authorized for association; and (3) association is a process to establish a mapping between an AP and a mobile station. Re-association occurs instead of association when the mobile station is already associated with another AP.
According to studies, scanning is the most time-consuming among the three components of the handoff, accounting for more than 90% of the overall handoff latency. The reason why scanning latency is high is because scanning was originally designed for the initialization of a mobile station. When a mobile station is initially switched on, the mobile station does not have any information about neighboring APs to associate with. The mobile station scans for neighboring APs to collect information about the neighboring APs.
Scanning methods may be classified into an active scanning method and a passive scanning method. In the active scanning method, a mobile station broadcasts a Probe Request frame on a predetermined channel and waits until an AP of the channel transmits a Probe Response corresponding to the Probe Request frame. If the mobile station does not receive the Probe Response during a MinChannelTime or a MaxChannelTime, the mobile station repeats the process using another channel.
In the passive scanning method, a mobile station listens for a beacon transmitted from an AP at predetermined time intervals and finds neighboring APs via information contained in the beacon. In the passive scanning method, scanning may be performed when the mobile station receives the AP's beacon. Since the mobile station must wait for the beacon, the passive scanning method has latency greater than the active scanning method.
FIG. 1 illustrates an example of a typical active scanning method according to IEEE 802.11 standards. A mobile station 100 broadcasts a Probe Request frame on a channel n−1 and waits for a Probe Response frame from an AP 150 for MinChannelTime. If no available AP 150 exists on the channel n−1, the mobile station 100, having waited for the MinChannelTime, determines that the AP 150 does not exist on a corresponding channel n−1, switches channels during ChannelSwitchingTime, and broadcasts a Probe Request on the channel n. APK and APK+1 operate on the channel n and transmit a Probe Response to the mobile station 100 in response to the broadcast. The mobile station 100 receives the Probe Request from APK within MinChannelTime. If the channel n is busy, the mobile station 100 extends a wait time for the Probe Response up to MaxChannelTime. After searching for the AP 150 on the channel n, the mobile station 100 switches a channel during ChannelSwitchingTime and searches for the AP 150 on a channel n+1.
According to the scanning method of IEEE 802.11 WLANs illustrated in FIG. 1, a scanning failure occurs when the mobile station 100 does not receive even a single Probe Response from the AP 150 despite scanning all channels. A shorter MaxChannelTime increases the probability of a scanning failure. Similarly, a longer MaxChannelTime decreases the scanning failure probability, but increases an overall scanning latency. This creates a trade-off between scanning failures and scanning latency. Many studies have been made on an optimal solution between the scanning failure and the scanning latency, but these studies only suggest a value of MaxChannelTime/MinChannelTime. One such example is discussed in “Techniques to reduce the IEEE 802.11b handoff time” in proceedings IEEE ICC '04, June 2004. A technology which can reduce both scanning failure and scanning latency has not been suggested until now.